Methods for the reduction and sintering of bodies containing reducible metal compounds



Patented June 3, 1952 parse s'r METHODS FOR THE REDUCTION AND SIN-TERING OF BODIES CONTAINING REDUC- IBLE METAL COMPOUNDS of Sweden NoDrawing. Application February '28, 1948, Se-

rial No. 12,006. In Sweden February 8, 1947 2 Claims. (01. 75-22) Theinvention is for improvements in and relating to the reduction andsintering of bodies containing or consisting of reducible metal.compounds.

One object of the invention is to enable the reduction and sintering ofsuch bodies to be performed without formation of cracks or fissures inthe bodies. Another object is to adapt this improved method of reductionand sintering for the production of articles of predetermined shape andsize, finished or semi-finished, direct from reducible metal compounds.

According to the U. S. patent specifications Nos..2,386,072 and2,386,073 blocks are prepared from ores or metal oxides in the powderedform, embedded in carbonaceous material, heated and reduced to ironsponge. The British specification No. 3926/1897 describes the productionof finished articles direct from the ore. The ore is pressed to shapeand the bodies thus obtained are reduced and sintered in a reducing gasatmosphere.

According to the methods disclosed in, the U. S. Patent specificationsthe reducible material is embedded in and is on all sides supported'bythe carbonaceous material. The mechanical pressure of the reductionagent acts on the glowing, porous mass which during the reduction hassuch a high temperature as to assume a very plastic state and thereforeunder the pressure of the surrounding reduction agent is somewhatcompressed. It is therefore hardly possible for cracks to be formed inthe body, and even if such cracks are formed they are withoutsignificance, as iron sponge is being produced which is to be subjectedto further treatment.

It is different with the method disclosed in the British specification.In this case any cracks formed in an early stage of the reduction arenot closed up by any external mechanical pressure but remain open. Thisapplies of course to the sides and the top of the body. Cracks at thebottom on the other hand may be closed up under the pressure of thematerial above.

It has, however, been found that this difliculty in carrying out thereduction, particularly in a gas atmosphere, may be overcome if thefollowing procedure is adopted. The reduction is carried out'in two ormore steps in each of which the temperature is increased and for a 2predetermined period of time maintained within predetermined limits forpermitting the escape of the gases evolved by the reduction process totake place so slowly as to avoid formation of cracks in the body beingtreated.

Investigations of the relationship of time to temperature have shownthat for each oxide (oxides of the same metal but in diiferent stages ofvalency behave as different oxides) there are specific temperatureranges in which the evolution of gas during the reduction reaches apoint where distortion of the body being treated is liable to occur.

The drying and the reduction of the bodies may be eifected in acontinuous process. The reduction is carried out at temperatures whichare far below the temperatures used for the production of 'metals by theusual melting methods.

The manner in which the reduction is carried out therefore is of thegreatest importance for the success of the process. In order toillustrate this the reduction of iron oxide (magnetite ore) will bedescribed in detail. It is assumed that finely divided magnetite ore iscompressed to a body having substantially the shape and dimensions ofthe final article to be produced. The compact is dried at a temperatureof to 200 C. and can then be heated comparatively quickly toa'temperature between 425 and 475 C. At this temperature (theprereduction temperature) and the reduction velocity for FezOwFeO(magnetite ore=Fe3O4=FezO3, FeO) is fairly high but not so high that thereaction products formed burst the very brittle mass. If now thetemperature is quickly raised above this point the reduction velocityincreases to such an extent that the mass no longer retains its shapebut numerous cracks are formed. Therefore, after reaching thetemperature 425 to 475 C. this temperature must be maintained for apredetermined period of time which depends on the dimensions of thebody. The temperature may then be raised, but only very gradually, up to500 0., and after that more quickly to a temperature of about 600 to 650C. At this temperature a new reduction process sets in which is verylively and accompanied by the evolution of large quantities of gas, asFeO is reduced to Fe. Also in this case the temperature must at first bekept constant, at approximately 600 to 650 C., and can then be raisedslowly up to about 700 C. From this point the temperature may be raisedmore quickly to the temperature, 1000 to 1250 C., at which the finalreduction and an appreciable sintering of the mass occurs. Thissintering is accompanied by shrinking and an increase of the strengthproperties.

It is of the greatest importance that the reduction closely follows apredetermined temperature-time scheme which depends on the nature of thereducible metal compound, the dimensions of the body being treated etc.The bodies which are passed into the furnace must when they pass out ofthe. furnace finally reduced and sintered be free from cracks in orderthat the subsequent treatment may be carried out successfully.

The consumption of energy corresponds to approximately 500 kilograms ofcoal per ton iron (according to Wibergs method for the production ofiron sponge). In this figure the energy consumption for maintaining thereduction temperature as Well as the amount of reduction agent requiredfor the reduction itself is included.

This description relating to iron ore applies generally to any otherreducible oxide, but the temperatures vary from case to case and so mayalso the time periods. Thus for examplecopper oxide requiresconsiderably lower reduction temperatures than iron oxides.

If different reducible oxides are present in comparatively large amountsit may be necessary to keep the temperature constant or raise thetemperature very slowly in many different steps. It is also necessary tohave regard tothe different valencies of the different oxides. In mostcases, however, oxides requiring reduction temperatures substantiallydifferent from that of the mainbody are present only in small amounts,and in such cases it has been found unnecessary to take the conditionsof reduction of these oxides into consideration. In such cases it isthus possible to conduct the process considering only the nature ofthese oxides which constitute the main body of thematerial beingtreated.

As reducing agent may be used reducing gases from an external source,and in a continuously workingfurnace these gases preferably areconducted in opposite directionto that in which the bodies being treatedare passed through the furnace. In some cases, however, particularly ifthe bodies to be treated have large sectional areas which render thediffusion of the reducin gases through the porous mass difficult or evenimpossible, it is preferable to mix a solid reducing agent, for examplecarbon, carbonaceous products, tar etc., with. the material to bereduced. In such cases it may be preferable to use. an amount of carbonwhich is sufficient also to carburize the product subjected to finalreduction and final sintering, so that a product having the desiredcarbon content is, obtained. The reduction may also be carried out byusing both methods.

It is also possible to produce alloyed semimanufactured or finishedproducts by reducing mixtures of oxides or other reducible compounds.Also in these cases the reduction is generally carried out at atemperature below the melting point of the metal having the lowestmelting point. On the other hand it is not always possible to adhere tothis rule if a component having a particularly low melting point ispresent.

The reduction as above described may be car ried out in chamberfurnaces, bell furnaces or furnaces of the type in which the goods iscontinuously passed through the furnace. When using chamber furnaces thetemperature may with advantage be controlled by means of programmeregulators. The continuous furnace is particularly advantageous for theproduction of profiles, bands, tubes etc. of great lengths.

A continuous furnace preferably suitable for the process of theinvention should have a number of different zones, i. e. one zonepermitting a comparatively slow increase of the temperature from about70to C. to about 200 C. for drying the body being treated. Thetemperature can thenbe raised comparatively quickly to the pointcorresponding to. the first point of inflection on the curve obtained-byplotting the data obtained by a thermal analysis of the reductionprocess of the material to be treated (in the following this point willbereferred to as the first reduction maximum, whereas the temperature atwhich the reduction process FeO Fe becomes more lively willbe called thesecond reduction maximum). The temperature corresponding to the firstreduction maximum is maintained'constant.forsuch a period of time thatthe evolution of'gas which accompanies thev reduction proceeds quietly,so that bursting of or the formation of fissuresin the mass is avoided.The temperature is then raised, atfirst very slowly. and thenmorequickly, until. the second reduction maximum is reached; When thistemperature has been reached. there follows a zone in the furnace inwhich the temperature ismaintained constant, and. then the temperaturemay again, .at first slowlyand then comparatively quickly, be raised to.the desired end temperature at which the reduction and sintering iscompleted. The process. described above apply to the reduction of ironore with hydrogen. If other reducing agents or combinations of differentreducing agents are employed other temperatures will have to be used-butthe principle remains the same.

As final product after the reduction a porous metal body of low strengthand having a pore volume of the order of 30 to 70% is obtained. Suchmetal articles may in some cases be used as they are; they can withadvantage be used asgas or liquid filters.

An important application field of the method of the invention is,however, the production'of compacts. For this purpose the reductioninsteps is followed by presintering and final reduction in a further stepat astill higher temperature, and then follows compacting andfinalsintering.

The compacting operation my consist of" such a. working of the porousmaterial. as. forging, pressing, hammering, rolling and so on in; thehot-or cold state. In the case of steel for example it is possible tocontinue heatingthe bodyreduced at a temperature of 1000 C. in the samefurnace in which the sintering is carried out to a temperature suitablefor rolling and pass the body from the furnace straight to a rollingmill, a hammering; machine or the like, so that the body is immediatelycompressed to the compact state. Then follows an annealing treatment orfinal'sintering at a temperature of 1000 to-1250 C., and a ma terial isobtained having a pore volume of 3 (305% and a strength which is notappreciably different from that of molten and rolled steel. As thedesired result is obtained by meansof one or at the most two rollingoperations or one or two forging operations the energy consumptionis-only 30% of that required for the normal reducing rolling ofcastings.

It is of importance that the final reduction takes place at such a hightemperature as to carry the sintering of the material to such a pointthat the strength of the material is sufiicient for compression directlyafter the sintering process.

The invention also comprises a method for the production of articles ofpredetermined shape and dimensions direct from reducible metalcompounds, using the reduction method above described. Thismethodconsists in preparing from the powder of the reducible metalcompound a body having substantially the shape of the finished article,drying the body, reducing the metal compound in two or more stages ineach of which the temperature is increased and for a predeterminedperiod of time maintained within predetermined limits so as to permitescapeof the gases evolved by the reduction to take place so slowly asto avoid formation of cracks in the material, completing the reductionand presintering at a still higher temperature, compacting to finalshape and resintering.

From the U. S. patent specifications Nos. 2,386,072 and 2,386,073referred to above it is known to produce iron sponge from ores andoxides in the powdered form by mixing the powder with a binding agent,forming a compact mass, surrounding the formed body with a carbonaceousmaterial, and for a period of time of 4 to 5 hours, heat to reducingtemperature (approximately ll00 0.), the reduction being effected bymeans of reducing gases generated by the carbonaceous material.

The method of the present invention difiers from the known methods justdescribed in that it is a question of producing articles of final shapeand sintered to completion, not iron sponge, that the reduction is noteffected by packing in a solid reducing agent but in a free gasatmosphere, by means of a reducing agent contained in the body beingtreated or by both methods, that the reduction takes place in two ormore steps in each of which the temperature is increased and accordingto a predetermined scheme, and that the reduction and sintering isfollowed by compacting and final sintering. It may be added that whenemploying the methods disclosed in the U. S. patent specifications therate of diffusion of the reducing gases formed during the heating islimited and that therefore the production by these methods of bodieshaving thick walls is rendered difilcult and uneconomical.

The British specification No. 3926/ 1897 relates to the production ofbars and articles of final form direct from the ore. The ore is finelydivided, mixed with water or tar and coal dust, and from the paste thusobtained are moulded the articles to be made. in a reducing atmosphereto a temperature below the melting point of the metal of which theobjects will be made up. The method of the invention differs from theknown method in that the reduction is carried out in two or more stepsand the sintering in two steps between which is interposed a compactingoperation.

Generally speaking, powder metallurgical methods are known according towhich metal powder is produced one way or the other, is pressed to shapeand sintered. From these -methods the invention differs in that articlesof final shape are made in a continuous process direct from the rawmaterial.

The method described above for the production The objects are heatedtons per square centimeter.

of articles of final shape direct from the raw material may be used forthe production of semimanufactured articles such as bars, sheet metal,tubes and the like, and for the production of finished articles.

The raw material may consist of highly concentrated ores, pure oxides,sulphides, calcined pyrites and so on. The powder is moulded into bodieshaving the shape of the final article to be produced. The moulding maybe efiected by compressing the dry powder but it is preferable tobring\the powder into a more or less plastic paste by means of asuitable process. Thus it has been found that by adding 3 to 15% of coaltar finely divided and highly concentrated magnetite ore which in itselfis very short and absolutely unplastic acquires very good plasticproperties. In another case it was found possible to plastify a powderediron oxide by adding a very small amount of freshly precipitated ironhydroxide. Calcined pyrites could be plastified by adding 2% of manganichydroxide. By adding freshly precipitated iron hydroxide to a mixture.of copper oxide and iron oxides a sufficiently plastifying effect wasobtained. Also other oxihydrates and other plastifying methods may beused.

The energy consumption for the plastification is very small andcorresponds at most to 50 kilograms per 1000 kilograms of metal.

The moulding to bars, tubes, bands, profiles etc. may be efiected bycompression in press dies as in the plastic industry or by extrusion asin the production of plastic masses or in the extrusion of easilymouldable metals. The plastic mass may also be rolled out in rollingmills. Thus for example the plastic mass may be extruded on a carrier ofordinary paper which is passed between two rollers. This method isparticularly suitable for the production of band or sheet. The carrierof paper can be disregarded as it is carbonized during the subsequentheat treatment. Whatever method is used for shaping'the plastic massesthe compression pressures required are low and in any case are less than3 The energy consumption is therefore small and does not exceed a valuecorresponding to 25 kilograms of coal per ton of metal.

The drying, reduction, presintering, compression and final sinteringthen is carried out as above described. The drying operation may,however, be carried out in some other way, for example at leastpartially in porous moulds. The presintering is generally effected at atemperature which is 50 to 150 C. below the final sintering temperature,and the final sintering is generally carried out at a temperature whichis not lower than of the absolute melting temperature of the metalconcerned.

When producing profiles, bands, tubes, etc, in continuous lengths it maybe preferable to admix the reducing agent, the whole amount or a portionthereof, in th solid state with the mass to be reduced (large crosssections), or in some cases (small cross sections) to reduce only bymeans of reducing gases.

If the compression, forging, hammering, rolling etc. is carried out inimmediate continuation of the presintering operation there is no risk ofoxidation of the interior of the metal product, as the interior of theporous metal body is filled with reducing gases which during the workingare pressed out through the pores.

The shrinking during the combined reduction and sintering process hasgenerally been found to be of the order of to of the original dimensionsof the formed body. In the subsequent compression operation the volumeof the object was reduced by a further to Below is a summary comparisonof the energy consumptions required for the production of band steelaccording to the usual melting method and according to the powdermetallurgical method of the invention above. described. For theconversion of coal' to kilowatt-hours it has been as sumed that 1kilogram coal corresponds to 3 kilowatt-hours when the coal is used forgenerating heat. For the sake of simplicity the same conversion figurehas been used for the carbon required for the reduction of ore or ironoxides to metal.

The figures indicatekilowatt-hoursper ton Fe.

P'rod'ucticm by melting Kilowatt-"hours per ton Production of pig iron 12,100 Production of open hearth iron or electrosteel from pig-iron andscrap iron 600 Heating of castings and semi-manufactured products torolling temperature -1 500 Rolling 200 Total. Fe 3,400

Powder metallurgical production Kilo'watt-hour's per ton Plastifying theraw materials 150 Reduction, sintering and heating to rollingtemperature 1,500 Rolling '75 Total, Fe 1,725

assumed to cancel out each other, if anything theadvantage is to the newmethod described.

The-invention will be illustrated by the following examples withoutbeing limited thereby.

Example 1 Powdered iron ore was mixed with 3% coal tar and compressed ata pressure of 4 tons per square centimeter to cylindrical compacts. Thecom pacts were dried at a temperature of 200 C. and thenallowed to cool.The compacts were then heated in a continuous furnace in a hydrogenatmosphere to 500 C. at a rate of heating of 5 C. per minute. Thetemperature was then raised so as to reach 600 C. at the end of.approximately 30 minutes and at this temperature the compacts werereduced during a period of tim of 3 hours. The temperature was thenraised as quickly as possible to 1250 C. this temperature was maintainedfor 30 minutes. The compacts were. allowed to cool and the totalshrinking of the ore volume was found to be 2% and the porosity about50%. The reduced compacts were repressed in the original die to aspecific weight of 7.0 grams per cubic centimeter and sintered ata-temperature of 1250 C. for a period of time of one hour. The tensilestrength was then found to be 18 kilograms per square millimeter.

Example 2 Compacts prepared as in Example 1 were slowly I centimeter anda tensile strength of 10 kilograms per square millimeter.

Example 3 Highly concentrated iron ore was ground to a particle size ofless than" 0.3- millimeter and in-'- timately mixed with 20 of a 10% waxemulsion to a plastic mass. The water in the emulsion was evaporated andafter treatment in a kneading machine a powder having good nowproperties was obtained. It also had good plastic properties whensubjected to high pressure. As a result of the good flow properties thedetermination of the amount required for the dies was easy.

The powder was formed by extrusion into a tube having an internaldiameter of 25 millimeters, a wall thickness of 12.5 millimeters and alength of 1 meter.

The drying and the reducing sintering was car ried out as in Example 2.After final sintering the tube was taken directly to a hammering machineand compressed hot over a mandrel having a diameter of 23millimeters andthe hammer heads so adjusted that the outer diameter of the tube wasreduced to 36 millimeters. In this way a tube was obtained having alength of 2meters, a density of 7.6 and a tensile strength of -28kilograms per square millimeter.

What we claim is:

1. In a process for producing an article of predetermined shape frompowdered material consisting essentially of F6203, the steps comprising.compacting said powdered material at a pressure of not over about 3 tonsper square centimeter into a body having. substantially the shape of thearticle to be produced, heating the compacted body at a rate notexceeding 5 0. per minute to a temperature of about 600 C. in anatmosphere consisting essentially of a reducing atmosphere. andmaintaining it at said temperature fora sufficientlength of time for asubstantial partof the gas evolved at said temperature to escape duringthe reduction of the'valency involved without causing cracks in thebody, and then heating to a temperature in the range of 1000 to 1250 C.in an atmosphere consisting essentially of a reducing atmosphere for asumc-i'ent-length of time to complete the reduction of the oxide and forat least presintering to take place.

2. In a process for producing an article of predetermined shape frompowdered material consisting essentially of Fe203 with a small amount offreshly precipitated iron hydroxide, the steps comprising compactingsaid powdered material at a pressure of not over about 3 tons per squarecentimeter into a body having substantially the shape of the article tobe produced, heating the compacted body at a rate not-exceeding 5 C. perminute to a temperature of about 600 C. in an atmosphere consistingessentially of a reducing atmosphere, and maintaining it at saidtemperature for a sufficient length of time for a substantial part ofthe gas evolved at said temperature to escape during the reduction ofthe valency involved without causing cracks in the body, and thenheating to a temperature in the range of 1000 to 1250 C. in anatmosphere consisting essentially of a reducing atmosphere for asufficient length of time to complete the reduction of the oxide and forat least presintering to take place.

SVEN INGVAR HULTHENi MATII HAAKON AUGUST TIKKANEN.

10 REFERENCES CITED The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date Kuzel Aug. 11, 1908 Ladoflz Oct.8, 1912 Liebmann Sept. 22, 1914 Musso Mar. 21, 1933 Sherwood Dec. 29,1936 Clark July 18, 1939 Volterra Mar. 30, 1943

1. IN A PROCESS FOR PRODUCING AN ARTICLE OF PREDETERMINED SHAPE FROMPOWDERED MATERIAL CONSISTING ESSENTIALLY OF FE2O3, THE STEPS COMPRISINGCOMPACTING SAID POWDERED MATERIAL AT A PRESSURE OF NOT OVER ABOUT 3 TONSPER SQUARE CENTIMETER INTO A BODY HAVING SUBSTANTIALLY THE SHAPE OF THEARTICLE TO BE PRODUCED, HEATING THE COMPACTED BODY AT A RATE NOTEXCEEDING 5* C. PER MINUTE TO A TEMPERATURE OF ABOUT 600* C. IN ANATMOSPHERE CONSISTING ESSENTIALLY OF A REDUCING ATMOSPHERE, ANDMAINTAINING IT AT SAID TEMPERATURE FOR A SUFFICIENT LENGTH OF TIME FOR ASUBSTANTIAL PART OF THE GAS EVOLVED AT SAID TEMPERATURE TO ESCAPE DURINGTHE REDUCTION OF THE VALENCY INVOLVED WITHOUT CAUSING CRACKS IN THEBODY, AND THEN HEATING TO A TEMPERATURE IN THE RANGE OF 1000 TO 1250* C.IN AN ATMOSPHERE CONSISTING ESSENTIALLY OF A REDUCING ATMOSPHERE FOR ASUFFICIENT LENGTH OF TIME TO COMPLETE THE REDUCTION OF THE OXIDE AND FORAT LEAST PRESINTERING TO TAKE PLACE.